The present invention relates generally to a magnetic resonance imaging (MRI) scanner and more particularly to a low acoustic noise MRI scanner.
MRI scanners, which are used in various fields such as medical diagnostics, typically create images based on the operation of a magnet, a gradient coil assembly, and a radiofrequency coil(s). The magnet creates a uniform main magnetic field that makes unpaired nuclear spins, such as hydrogen atomic nuclei, responsive to radiofrequency excitation via the process of nuclear magnetic resonance (NMR). The gradient coil assembly imposes a series of pulsed, spatial-gradient magnetic fields upon the main magnetic field to give each point in the imaging volume a spatial identity corresponding to its unique set of magnetic fields during an imaging pulse sequence. The radiofrequency coil applies an excitation rf (radiofrequency) pulse that temporarily creates an oscillating transverse nuclear magnetization in the sample. This sample magnetization is then detected by the excitation rf coil or other rf coils. The resulting electrical signals are used by the computer to create magnetic resonance images. Typically, there is a radiofrequency coil and a gradient coil assembly within the magnet.
Magnets for MRI scanners include superconductive-coil magnets, resistive-coil magnets, and permanent magnets. Known superconductive magnet designs include cylindrical magnets and open magnets. Cylindrical magnets typically have an axially-directed static magnetic field. In MRI systems based on cylindrical magnets, the radiofrequency coil, the gradient coil assembly and the magnet are generally annularly-cylindrically shaped and are generally coaxially aligned, wherein the gradient coil assembly circumferentially surrounds the radiofrequency coil and wherein the magnet circumferentially surrounds the gradient coil assembly. Open magnets typically employ two spaced-apart magnetic assemblies (magnet poles) with the imaging subject inserted into the space between the assemblies. This scanner geometry allows access by medical personnel for surgery or other medical procedures during MRI imaging. The open space also helps the patient overcome feelings of claustrophobia that may be experienced in a cylindrical magnet design.
A gradient coil assembly comprises a set of windings that produce the desired gradient fields. Such an assembly for a human-size whole-body MRI scanner typically weighs about 1000 kg. The windings consist of wires or conductors formed by cutting or etching sheets of conducting material (e.g. copper) to form current paths to generate desired field patterns. The wires or conducting coils or plates are themselves typically held in place by fiberglass overwindings plus epoxy resin.
Generally, the various components of the MRI scanner represent sources and pathways of acoustic noise that can be objectionable to the patient being imaged and to the operator of the scanner. For example, the gradient coil assembly generates loud acoustic noises, which many medical patients find objectionable. The acoustic noises occur in the imaging region of the scanner as well as outside of the scanner. Known passive noise control techniques include locating the gradient coil assembly in a vacuum enclosure.
Large pulsed electrical currents, typically 200 A or more, with risetimes and durations typically in the submillisecond to millisecond range, are applied to the windings. Because these windings are located in strong static magnetic fields (e.g., 1.5 T for a typical clinical imager to much higher values for research systems), the currents interact with the static field and strong Lorentz forces are exerted on different parts of the gradient coil assembly. These forces in turn compress, expand, bend or otherwise distort the gradient coil assembly. It will be readily understood by those skilled in the art that the frequencies of the acoustic noise so generated will be in the audio range. Typically there are strong components of noise from 50 Hz and below to several kHz at the upper end of the frequency range.
Vibrations can be conveyed mechanically from the gradient coil assembly to the patient area, the cryostat or other external parts of the MRI scanner via the gradient coil assembly support frame. These transmitted vibrations can cause the external parts to vibrate and thereby produce acoustic noise, which will be heard by the MRI subject and the MRI operator.
One way to decrease the transmitted vibrations is to use a passive vibration isolation mounts for the gradient coil assembly. It is known in the mechanical arts area to design and use isolation mounts so that vibrations from machinery supported by the isolation mounts are not transmitted to surrounding structure that supports the isolation mounts. Conventional isolation mounts include those of the elastomeric type and those of the spring type. Such isolation mounts are designed such that the natural frequency of vibration of the mounts and the machinery is less than the important excitation frequencies of the machinery in order to provide effective vibration isolation.
In one approach to providing a vibration isolation mount, solid metal brackets are mounted on the gradient coil assembly and corresponding solid metal brackets are attached to the cryostat. The gradient coil assembly is positioned so that the brackets are aligned and elastomeric pads (for example, rubber) are positioned between each cryostat bracket and the corresponding gradient coil assembly bracket. With this configuration, the transmission of vibration from the gradient coil assembly to the cryostat is attenuated by the elastomeric pads.
Unfortunately, there is a limit to the degree of passive attenuation achievable by use of elastomeric pads or spring isolation mounts as described. Generally speaking, softer pads or springs produce greater attenuation. However, the pads or springs underneath the gradient coil assembly must be able to support the gradient coil assembly weight. Also, pads or springs that are too soft might permit excessive motion of the gradient coil assembly in response to Lorentz forces, in which case image quality could be adversely affected. Pad or spring stiffness is thus a tradeoff between keeping the gradient coil assembly precisely positioned, on the one hand, and attenuating vibration transmission on the other.
In another approach active vibration compensation is used to substantially improve the vibration isolation efficacy as described above. One example of this approach is disclosed in Roozen et al., U.S. Pat. No. 6,549,010, 2003. FIG. 1 is a reproduction of part of this device, and shows a gradient carrier 18 supported by suspension elements 19 including a resilient element 22 and an active drivable element 21 connected in series with suspension element 22. If forces cause a displacement of gradient carrier 18, then drivable element 21 can be lengthened or contracted to counteract the displacement and thereby prevent any force being applied to suspension element 22. Unfortunately, with this arrangement, active element 21 must be able to withstand the entire weight of the gradient carrier, which may be hundreds of pounds. Roozen et al. describe drivable element 21 as a piezo actuator. However, such actuators are fragile and could fracture in this configuration.
These approaches to reduce acoustic noise due to the various components in MRI scanners have been partially effective, but patients and technicians still find the noise in and about a MRI scanner to be problematic. What is needed is a lower noise MRI scanner that addresses the multiple sources and pathways of acoustic noise in and about the scanner.